This invention relates to a method of forming contacts. More particularly, this invention relates to a method of forming contacts between a ceramic multilayer circuit board and a metal support substrate.
Low temperature firing multilayer ceramic circuit boards are known that are suitable for use with low melt temperature conductive metals, such as silver, gold and copper. They have a low thermal coefficient of expansion (TCE) and they may be formulated to be compatible with both silicon and gallium arsenide devices.
These ceramic circuit boards are made from glasses that can be fired at low temperatures, e.g., temperatures of less than 1000xc2x0 C. The circuit boards are made by admixing finely divided selected glass particles or powders, and optional inorganic fillers, with organic materials including resin, solvents, dispersants and the like. The resultant slurry is cast as a thin tape, called green tape. A circuit pattern may be screen printed onto the green tape using a conductor ink formulation comprising a conductive metal powder, an organic vehicle and a powdered glass, generally the same glass as that used to make the green tape.
A plurality of green tapes having printed circuits thereon can be stacked together. In such case, via holes are punched into the green tapes which are filled with a conductive via fill ink to provide electrical contact between the circuits on the various green tapes. The green tapes are then aligned, laminated under heat and pressure, and fired to remove the organic materials and to devitrify the glass.
Recently, multilayer ceramic circuits boards have been adhered to a metal support board for added mechanical strength. A bonding glass can be used to coat the metal support and to provide adhesion between the support and the laminated ceramic layers. An added advantage to this method is that the bonding glass reduces shrinkage of the green tapes in the x and y dimensions during firing. Thus most of the shrinkage occurs in the z, or thickness, dimension. The result is that tolerances between the circuits and the via holes can be reduced.
The glasses used to make the green tapes must have a TCE matched to that of the metal support to prevent delamination or cracking of the fired glass. The TCE of the green tapes can be modified by the addition to the green tape slurry of various inorganic fillers including metal oxides and metal powders.
The multilayer circuit boards are mounted on the metal support substrate and fired in air at temperatures between about 700 to 1000xc2x0 C. This firing forms a thin oxidized dielectric layer on the metal support substrate, preventing good electrical contact between the metal support and the circuits on the multilayer ceramics.
Thus methods have been sought to form low resistance (1 ohm or less) contacts between the metal support substrate and a multilayer circuit board mounted thereon, and to prevent the formation of a dielectric layer on the support substrate that impedes the formation of low resistance contacts.
Metal support boards for supporting a multilayer ceramic circuit board can be processed to prevent formation of a dielectric layer between the metal support and the overlying multilayer ceramic circuit board having circuitry and vias formed of a low melt temperature conductive metal. A patterned layer of a conductive metal, such as silver and/or gold, is formed on the unfired metal support. When the low melt temperature conductive metal used in the ceramic circuit board is made of silver, the patterned layer, if made of gold, can be covered with silver, as by screen printing with a fritless silver ink. The resultant contact pads can be connected to the circuits made of the same metal on the multilayer ceramic board. A silver screen printable ink including a glass frit can be applied directly to the surface of the metal support. The contact pads are then fired.
Just prior to mounting the laminated multilayer circuit board to the metal support, a conductive metal thick film ink is applied to the contact pads. The conductive metal is chosen to match the metal of the circuits and via fill inks of the circuit board.
The problem addressed herein is how to prevent formation of a dielectric layer on a metal support substrate which interferes with good electrical contact, i.e., to obtain less than 1 ohm resistance, between the metal support substrate and a multilayer ceramic circuit board.
Depending upon the metal chosen for the metal support, which can include copper clad molybdenum, copper clad Kovar(copyright), an alloy of iron, nickel, cobalt and manganese supplied by Carpenter Technology, titanium and the like, the ceramic is chosen so that it has a TCE match to the metal support. Various glasses and ceramic mixtures are known that have a TCE match to various metal support substrates.
The present invention will be illustrated using a nickel plated copper clad molybdenum support substrate, and printed circuits utilizing silver circuitry and silver via fill inks, but the invention is not meant to be limited thereto.
In accordance with the invention, several methods can be used to apply one or more contact pads onto the metal support board so that no dielectric layer is formed on the metal support substrate during the various known steps for assembling the various printed circuits. Such ceramic circuits can include embedded devices such as capacitors and resistors for example. In all cases, the metal support must be cleaned so as to be free of dielectric materials on the surface of the support.
Silver contact pads can be made using a two step method which includes first electrolytically depositing a soft gold layer, such as from a plating bath supplied as Technic Orotemp 24 by Technic Inc. The plating bath can be used with a mask layer on the support substrate, thereby depositing a patterned gold layer thereon directly, and then removing the mask layer. Alternatively an unpatterned gold layer can be deposited from the plating bath and the metal layer etched back through a mask to remove the unwanted gold. A patterned gold layer is thus formed with gold pads where future electrical connections are to be made.
A fritless silver thick film ink is then applied by screen printing so as to form silver pads over the gold pads. Generally, the silver pads should be slightly larger than the underlying gold pads, again to prevent formation of a dielectric layer during the subsequent firing step.
The patterned metal support is then dried and fired in air or nitrogen, as by inserting the support into a furnace. The temperature is increased over a period of about one hour to a peak temperature of between about 650 to 900xc2x0 C.
In an alternate method, gold dots, e.g., about 10 mil in diameter, can be screen printed onto the clean metal support substrate using a fritless gold thick film ink. This patterned gold layer is then dried, and fired in nitrogen using an appropriate thick film firing profile, to a peak temperature of about 650xc2x0 C. By appropriate firing profile is meant that the temperature is raised stepwise, first to remove the organic materials in the thick film ink, and then to complete firing of the metal layer.
A fritless silver thick film ink is then screen printed onto the fired support substrate over the gold dots. This silver layer is then dried, and fired in air or nitrogen over about an hour to a peak temperature of about 650 to 900xc2x0 C. to remove the organic materials and sinter the silver particles together.
In still another method, silver contact pads can be screen printed directly onto the metal support substrate using a glass-filled silver contact ink that includes a reducing agent. This ink is dried and fired in air over a one hour period to a peak temperature of about 800 to 850xc2x0 C.
The resultant fired metal support substrates include contact pads which can be connected electrically to overlying multilayer printed circuits, but do not have a dielectric layer under the contacts. A bonding glass layer, such as has been disclosed by Prabhu et al in U.S. Pat. No. 5,581,876, can be deposited over the remaining portion of the support substrate, by screen printing a bonding glass ink, thereby avoiding deposits over the contacts that would interfere with the low resistance contact pads.
After the patterned silver layer has been applied to the support substrate and fired, the prepared multilayer green tape stacks, having conductive circuitry on their surface and filled conductive vias to provide electrical connections between the circuitry on the green tapes, are aligned and laminated under heat and pressure using conventional techniques. Just prior to mounting the laminated green tapes onto the support substrate, a fritless silver thick film ink is screen printed so as to deposit a fresh silver layer onto the already formed contact pads.